There has sometimes been used an installation for the production of ammonia, which treats extremely high flow rates of synthesis gas, to withdraw a moderate flow rate of synthesis gas so as to elaborate hydrogen and, for this purpose, while compressing in several compression stages the synthesis gas to send it to an ammonia synthesis reactor to produce the flow of synthetic ammonia, there is withdrawn, between two compression stages, a relatively low flow rate of withdrawn synthesis gas under medium pressure, the hydrogen is substantially separated from the other constituents: nitrogen, argon, methane, to produce a flow of hydrogen as pure as possible, said separation taking place generally by permeation delivering substantially under said medium pressure a stream consisting essentially of hydrogen and nitrogen with substantially all the argon and all the methane, which stream is returned to the ammonia synthesis gas under medium pressure and a permeation gas under lower pressure consisting essentially of hydrogen with a small content of nitrogen and traces of argon, methane and as the case may be water vapor. There has also been proposed effecting said separation by adsorption on a molecular sieve according to the process with cyclic pressure variation, called "pressure swing adsorption" or PSA, delivering on the one hand, under medium pressure, a production gas consisting essentially of hydrogen with traces of argon and traces of nitrogen, methane and as the case may be water vapor, on the other hand, under a lower pressure, a residual gas with higher content of hydrogen, a substantial content of nitrogen, and low contents of argon, methane and as the case may be water vapor.
With these processes, one is confronted either with poor quality hydrogen product, or poor energy use, because the nitrogen, which is found in the residual low pressure gas, must be recompressed at the withdrawal pressure of the synthesis gas. On the other hand, and this is particularly true in the adsorption process, this process leads to poor performance of the hydrogen production installation because of recycling of substantial quantities of argon which is hardly adsorbable.